1. Field of the Invention
The present invention relates to a wireless local area network (WLAN) system, and more particularly, to a method of transmitting and receiving a physical layer convergence procedure (PLCP) protocol data unit (PPDU) by a station (STA) in the WLAN system supporting a power save mode operation.
2. Related Art
With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to above 540 Mbps, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate.
The WLAN system supports an active mode and a power save mode as an operation mode of a station (STA). The active mode implies an operation mode in which the STA operates in an awake state capable of transmitting and receiving a frame. On the other hand, the power save mode is supported for power saving of an STA required to stay the active state for receiving the frame. An STA supporting the power save mode (PSM) can avoid unnecessary power consumption by operating in a doze mode when it is not a time period in which the STA can access to a radio medium. That is, the STA operates in the awake state only for a time period in which a frame can be transmitted to the STA or a time period in which the STA can transmit the frame. With the widespread use of the WLAN and the diversification of applications using the WLAN, there is a recent demand for a new WLAN system to support a higher throughput than a data processing rate supported by the IEEE 802.11n. A next-generation WLAN system supporting a very high throughput (VHT) is a next version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11 WLAN systems which have recently been proposed to support a data processing rate of above 1 Gbps in a MAC service access point (SAP).
To effectively utilize a radio channel, the next-generation WLAN system supports multi user-multiple input multiple output (MU-MIMO) transmission in which a plurality of non-access point (AP) stations (STAs) concurrently access to a channel. According to the MU-MIMO transmission, an AP can transmit a physical layer convergence procedure (PLCP) protocol data unit (PPDU) concurrently to one or more MIMO-paired STAs.
However, since STAs that operate in the power save mode operate by transitioning between the doze state and the awake state, MU-MIMO transmission of the AP is dependent on an operation state of MU-MIMO paired STAs. Therefore, the AP needs to provide information on a time period for MU-MIMO transmission to the MU-MIMO paired STAs. Accordingly, there is a need for a method of transmitting and receiving a PPDU by using a MU-MIMO transmission scheme in the WLAN system supporting the power save mode operation.